


Work-Unsafe Experiments

by grossferatu



Category: Undertale
Genre: Angst, Ectobiology (Undertale), Ectovagina, Extra Limbs, Gaster is basically Sans’s father, Hurt No Comfort, Incest, M/M, Non-consensual sex, Orifice Sex, Plot What Plot/Porn Without Plot, Rape, Skullfucking, Tentacles, but more his creator, ectopenis, unspecified AU
Language: English
Status: In-Progress
Published: 2020-09-11
Updated: 2020-09-10
Packaged: 2021-03-06 19:15:23
Rating: Explicit
Warnings: Rape/Non-Con
Chapters: 1
Words: 885
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/26394004
Author URL: https://archiveofourown.org/users/grossferatu/pseuds/grossferatu
Summary: Sans belongs to Gaster, and so do all his holes. Gaster takes advantage of the benefits of ownership.
Relationships: W. D. Gaster/Sans
Kudos: 31





	Work-Unsafe Experiments

**Author's Note:**

> Gaster isn’t a skeleton in this, he’s some kind of biological monster with limb-summoning abilities. Updated as I get the inspiration!

Sans isn’t panicking when Gaster comes into his cell. Nope, not at all. He’s the chill one, right, the laid-back, almost lazy skeleton. Paps is the one who has high-strung emotions, who feels excited... or afraid. Not Sans. He’s cool, he’s—

He barely manages to avoid cowering when the metal door clangs slightly as it opens, the imposing, lab-coated figure of W. D. Gaster filling the doorway. “How’s my favorite experiment?” he asks, his tone bright and pleasant and utterly nauseating, even as Sans feels a shiver of anticipating.

”oh, you know,” Sans says. “just boney.”

Gaster laughs softly. He finds Sans’s puns funny—hell, he even seems to find Sans fun to be around—but that doesn’t change his behavior, doesn’t make it any kinder. He’s already seated himself on the chair in Sans’s cell Sans isn’t allowed to sit in, pulling up his lab coat. He never takes off his lab coat, at least not around Sans, not even for this. 

His long, black tentacle cock uncoils from between his legs, the tip already dripping gray fluid onto the floor of the cell. His expression doesn’t change. 

“Come here,” Gaster says.

Sans decides, spur of the moment, to shake his head. He stays put. Gaster’s expression finally breaks into a grin, and Sans feels two hands grab him around the shoulder blades, dragging him across his cell before depositing him on his knees in front of the doctor. 

“I’m your father, Sans,” Gaster says. He drags his cock against Sans’s face, solid and slick against his bones. He expects the length to be forced into his mouth, so he shuts his teeth. At least then he’ll have to break something to get inside him. 

“Sans,” Gaster says. “You have more than one hole, and some of them you can’t shut.”

The tip of Gaster’s cock is blunt and wetter than the rest of it, and when it presses against Sans’s eye socket, his vision in that eye goes gray before completely sputtering out, the corresponding eye light winking off.

Gaster moans, finally, and Sans shudders. This is going to be a long, long night, he realizes, as the cock forces his jaw open from the inside. 

“What I’m most curious about,” Gaster says. Despite the moan, he still sounds almost entirely unaffected. “Is how it feels to you. Inside you. Because you’re hollow, Sans, and that’s just so intriguing.”

He finally manages to force Sans’s jaws open completely, and Sans watches from his only functioning eye as his cock snakes its way out of his mouth like some kind of mutant tongue. 

Gaster wraps a summoned hand around the tip of his own cock, and begins to stroke himself. Sans is a captive audience, forced to endure the sensation of every shudder and flex and writhe of Gaster’s cock inside his skull. These are not parts of himself he should have sensory input for, but to his horror the wet sensation almost starts to feel _good_. As he realizes this, he feels the tell-tale tingle of his magic appearing. It’s a pussy this time. Another hole for Gaster to violate.

Gaster’s cum is black and oily and coats Sans’ vertebrae in spurts, the tip spasming violently under Gaster’s ministrations. He doesn’t wait for his orgasm to finish before dragging his cock back out of Sans’s eye-socket, the full length of it swollen against Sans’s face. 

A second tentacle appears out of the same darkness under Gaster’s coat, this one even thicker than the first. Wrapping the smaller tentacle tightly around Sans’s spine, he forces the second in a loop through both his hip sockets, before plunging the flared, swollen tip inside of his pussy. 

Sans can’t help himself. His moans quickly turn to whimpers, the sensation of Gaster’s cock against his spine sending jolts of half-pleasurable electricity through his body, the other cock sliding deeper and deeper into his pussy. 

He can speak now, and he finally manages a weak, “Stop. Please... just... _please_ , Gaster...”

Gaster tuts. “How many times have I explained what you’re supposed to call me when I do this, Sans?” he asks.

”...Father. Please, father, please stop.” Sans flushes, humiliated, as another moan escapes his mouth with the words. Why are his hip sockets erogenous? 

“I don’t think so, Sans,” Gaster says. His touch is gentle against his skull. “You’re trembling with pleasure. I wonder how deep I can plunder that wonderful magic of yours, my boy? How far will you feel it?”

Sans couldn’t cry—he was a skeleton—but he wished he could. “Please...” He continued, despite knowing it was hopeless. “Father, I don’t like it.”

His hips betrayed them, spasming in pleasure as his magic tried to take even more of Gaster’s cock inside of it. 

“Well,” Gaster said. “If you don’t like it, then at least give me the curtesy of explaining how you feel.”

Sans whimpered. “It’s... pressure.” His breath hitched. “You’re so hard, sliding against my bones. Please!” He wasn’t sure what he was begging for, anymore. 

“Very good. And inside you?”

”I want to take you forever,” Sans said. “I want...” He might as well say it. Gaster would make him anyway. “I want your other cock inside me, too.”

Gaster laughed. “Excellent,” he said. “I am more than happy to oblige.”


End file.
